Extract Array of JSONObjects from a nested Json using GSON in java

Question

I have a nested JSON object in which one of the nested objects (keyed parameters) is an unknown object itself. The only known part is that it's an array of further objects.

The problem is that the contents of this object are completely unknown(dynamic) and so I can't define a class with some known variable names for it.

What I need to achieve is to extract and store all these dynamic key-value pairs. For example in the below code, for parameters I need to extract both alpha and then its contents type, and store them as a Map.

js is the JSON I'm trying to parse into my class NestedJsonParse.

public class NestedJsonParse {
    String name;
    Experiment experiment;
    Parameters parameters;

    NestedJsonParse(String name, Experiment experiment, Parameters parameters) {
        this.name = name;
        this.experiment = experiment;
        this.parameters = parameters;
    }

    public String getName() { return name; }

    public String getExpName() { return experiment.getName(); }

    public String getParams() { return parameters.getParamsAsStrings(); }

    @Override
    public String toString() {
        return "NSP{" +
            "name='" + getName() + '\'' +
            ", exp-name='" + getExpName() + '\'' +
            ", params='" + getParams() + '\'' +
            "========================================"  +
            "}";
    }

    public class Experiment {
        String name;

        Experiment(String name) {
            this.name = name;
        }

        public String getName() { return name; }
    }

    public class Parameters {
        List<JSONObject>  parameters;

        Parameters(List<JSONObject> parameters) {
            this.parameters = parameters;
        }

        public String getParamsAsStrings() {
            return "abc";
        }
    }

    public static void main(String[] args) throws Exception {
        String js = "{\"name\" : \"e0\", \"experiment\" : { \"name\" : \"e1\" },  \"parameters\" : [ {\"alpha \": { \"type\" : \"float\" } } ]  }";

        JSONParser parser = new JSONParser();
        Object obj = parser.parse(js);  // Passes

        Gson GSON = new GsonBuilder().create();
        NestedJsonParse nsp = GSON.fromJson(js, NestedJsonParse.class);  // Fails
        System.out.println("NSP : " + nsp.toString());
    }
}

This passes if the JSON does not contain a parameters block but fails otherwise with this error:

Caused by: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 206 path $.parameters

pirho · Accepted Answer · 2018-12-28 22:05:05Z

1

One simple option is just to deserialize parameters as an Object so no class Parameters at all. Your NestedJsonParse could (greatly simplified) be like:

@Getter @Setter
public class NestedJsonParse {
    private String name;
    private Experiment experiment;
    private Object parameters;
}

Then when you do

NestedJsonParse nsp = GSON.fromJson(js, NestedJsonParse.class);

your parameters in nsp could be:

null
com.google.gson.internal.LinkedTreeMap
ArrayList containing above mentioned or other ArrayList etc...

depending on what is in the JSON.

Based on what you find from parameters you can take appropriate action after deserialization.

answered Dec 28, 2018 at 22:05

pirho

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